Environmental concerns associated with the use of fossil fuels have stimulated efforts towards developing various gas sensors. Although existing sensing technologies based on solid electrolytes, oxide semiconductors, and field effect devices exhibit significant potential for sensing applications at intermediate temperatures, e.g., below 600° C., there are challenges associated with poor device stability and low selectivity. Moreover, these challenges are more apparent at temperatures above 600° C.
An alternative approach to gas sensing includes optical methods such as absorption spectroscopy. Optical sensing techniques are immune to electromagnetic noise, and inherently safer than their electrical counterpart since the sensing elements are isolated from the interrogating electronics, thus minimizing the danger of explosion in environments with flammable or explosive gases.
Experimental effort has been focused on the development of sensors that employ noble metal nanoparticles due to their unique optical properties. In particular, gold (Au) and silyer (Ag) nanoparticles exhibit a strong surface plasmon resonance (SPR) band whose shape and spectral position is not only highly dependent on the refractive index of the host medium but also on chemical interactions, e.g. catalytic reactions at the interface between the metal and the surrounding environment. Theoretical modeling of silyer nanoparticles surrounded by a CO matrix/medium at room temperature has been undertaken. Kreibig, U.; Vollmer, M.; Optical Properties of Metal Clusters; Springer, N.Y., 1995.
Recently, Haruta and coworkers demonstrated the sensing potential of Au nanoparticles dispersed in a copper oxide (CuO) matrix to carbon monoxide (CO), at concentrations ranging from 50 to 10,000 ppm (1 vol. %) in air up between 175° C. and 300° C. Ando, M.; Kobayashi, T.; Iijima, S.; Haruta, M. Optical CO Sensitivity of Au—CuO Composite Film by Use of the Plasmon Absorption Chang, Sensors and Actuators B-CHEMICAL 2003, Vol. 96, Iss. 3, pp 589-595. The sensing mechanism was related to changes in the refractive index of the matrix due to the partial reduction of the CuO grains upon exposure to CO.
The inventors of the present invention earlier studied the effect of annealing temperatures on the microstructure and optical properties of Y2O3-stabilized ZrO2—Au nanocomposite films which were presented at the Materials Research Society meeting in Sep. 2004. FIG. 1 displays x-ray diffraction (XRD) patterns for the evolution of the microstructure of Au—YSZ nanocomposite films as a function of annealing temperature. The XRD patterns are plotted as diffraction peak intensity versus diffraction angle 2θ for the range from 25° C. to 55° C. As can be seen in FIG. 1, two poly crystalline phases were detected, one corresponding to the tetragonal YSZ phase, and the other to the face centered cubic Au phase. In addition, the XRD peaks became sharper and more intense with higher annealing temperature, indicating an increase in the crystallinity, and hence a rise in the average size of both the YSZ and the Au crystallites. These trends are attributed to the availability of a larger thermal energy at higher annealing temperature to drive crystallite coalescence, growth, and realignment.
The average Au crystallite size was calculated from the Scherrer formula using the Au XRD (111) reflection. The results of this analysis are displayed in FIG. 2, which plots the average crystallite size for Au as a function of annealing temperature. FIG. 2 indicates that the average Au crystallite size exhibited a gradual rise with annealing temperature, from about 4.0 nanometers (nm) at 600° C. to about 8.0 nm at 800° C. to about 9.5 nm at 900° C. However, a marked increase of about 5.5 nm was observed as the annealing temperature was increased from about 900° C. to about 1000° C., indicating a potential change in the underlying mechanism that drives the coalescence and regrowth of the Au crystallites.
FIG. 3 displays RBS data for the spatial distribution of Au atoms versus film depth within the YSZ matrix. The data are plotted as elemental RBS peak intensity versus RBS channel, with the width and the height of each peak determined by the spatial distribution and relative concentration of the corresponding element, respectively. In this context, no significant change was observed in the height or FWHM of the Au RBS peak with respect to the Zr peak as a function of annealing temperature, indicating that the average concentration of Au atoms as a function of film depth is not affected by the annealing process.
These findings, when coupled to the increase described above in the average Au crystallite size with the rise in annealing temperature, imply that at temperatures below about 900° C., Au crystallites grow through a solid state diffusion mechanism of individual Au atoms through the YSZ matrix. Alternatively, above about 900° C., the annealing temperature approaches the melting point of Au. The latter is 1064° C. for bulk Au but has been shown to be significantly lower for Au in nanoparticle form. For instance, a melting point of about 900° C. has been reported for 10 nm silica-encapsulated Au particles.
Accordingly, it is believed that above about 900° C., the growth of Au crystallites is still governed by the diffusion of Au atoms through the YSZ matrix. However, in contrast to solid-state diffusion of individual Au atoms observed below about 900° C., the marked increase in Au crystallite size above 900° C. suggests the occurrence of Au crystallite growth via an Ostwald ripening process. In this process, larger Au crystallites with lower interfacial curvature grow at the expense of their smaller counterparts with higher interfacial curvature, via the migration of individual Au atoms. This suggestion is in agreement with previous studies on the growth mechanism of Au nanoparticles in a silica matrix.
With regard to the film optical properties (at room temperature and in air) as a function of annealing temperature, FIG. 4 illustrates a typical absorbance spectra of Au—YSZ nanocomposite films over the wavelength region from about 300 nm to about 800 nm. As can be seen in FIG. 4, an SPR band due to the light-induced collective, oscillatory motion of the conduction electrons of Au is prominently present around about 600 nm. The band maximum was observed to shift toward longer wavelengths or “redshift” and become sharper and more intense with higher annealing temperature.
There is a need for further optical gas sensors that can operate under harsh environments and at high temperatures.